Sahih Muslim 1963
Sahih Muslim · Book 35: The Book of Sacrifices · The age of sacrificial animals
Jabir reported Allah's Messenger (ﷺ) as saying:
حَدَّثَنَا أَحْمَدُ بْنُ يُونُسَ ، حَدَّثَنَا زُهَيْرٌ ، حَدَّثَنَا أَبُو الزُّبَيْرِ ، عَنْ جَابِرٍ ، قَالَ قَالَ رَسُولُ اللَّهِ صلى الله عليه وسلم " لاَ تَذْبَحُوا إِلاَّ مُسِنَّةً إِلاَّ أَنْ يَعْسُرَ عَلَيْكُمْ فَتَذْبَحُوا جَذَعَةً مِنَ الضَّأْنِ " .
Sacrifice only a grown-up animal, unless it is difficult for you, in which case sacrifice a ram (of even less than a year, but more than six months' age).